Pulsed electro-hydraulic calibration of stamped panels
A tooling method for calibrating a partially formed metal part in an electro-hydraulic forming tool. The method includes loading a partially formed metal part onto a forming surface of a die. The part is then clamped onto the forming surface and the part is pulsed with a high rate energy pulse to overcome a spring-back effect in the part. The EHF calibration tool includes a punch to which a part is held by clamps while a capacitive discharge circuit is discharged through electrodes provided in the tool. Upon discharge of the capacitive discharge circuit through the electrodes, a high rate energy pulse is applied through a liquid to calibrate the part.
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1. Field of the Invention
The present invention relates to sheet metal forming processes and tooling for reducing the effect of spring-back on formed panels.
2. Background Art
Sheet metal is generally formed in a sheet metal forming process in which a sheet metal blank is drawn to an initial shape, stamped, flanged, formed and pierced in a series of steps. Spring-back occurs as a result of bending moments that develop in the blank as the sheet metal is formed to the desired shape. Spring-back causes the panels to partially return to a prior shape after a panel is formed in a sheet metal die or other sheet metal forming process.
New types of materials have been proposed for making sheet metal parts with higher strength and lower weight. Specialized steels and aluminum sheets are available that offer high strength and low weight which is desirable in many applications. Many high strength and low weight metals are subject to increased spring-back after forming.
One approach to compensating for spring-back is to predict spring-back in the die design process. The shape of the die may be modified to compensate for spring-back.
Another approach to compensating for spring-back is to stretch the formed blank to eliminate bending moments in the blank. If a part is to be stretched to reduce spring-back, the depth of draw must be limited to permit the stretching operations to adequately compensate for spring-back.
The degree of spring-back may vary from coil to coil. Some coils have only limited spring-back, while other coils even of the same grade or alloy may have greater spring-back. Spring-back is also affected by the extent of wear of the sheet metal forming dies. Increased spring-back may occur when the dies become worn.
The above problems are addressed by Applicant's invention as summarized below.
SUMMARY OF THE INVENTIONAccording to one aspect of the present invention, a method of calibrating a partially formed metal part is provided. By the term “calibrating” Applicant means that the part is stretched or re-struck to cause the partially formed part to more closely correspond to the desired part configuration. The method includes the step of loading the partially formed metal part onto a forming surface of a die. The part is then clamped onto the forming surface so that exposed portions of the part on opposite sides of the part from the surface engaging the die are exposed. The exposed portions of the part are pulsed with a high-rate energy pulse to overcome a spring-back effect in the part.
According to other aspects of the invention, the loading step may further comprise loading the part into an electro-hydraulic forming tool. In the pulsing step, an electro-hydraulic forming pulse is imparted to the panel.
According to other aspects of the present invention, the clamping step may be performed with a plurality of clamps that engage the part at spaced locations during the time that the high energy rate pulse is applied to the exposed portions of the part. The clamps may be repositioned as a second high energy rate pulse is being applied to the newly exposed portions of the part. Alternatively, the clamping step may be performed with a reticulated clamp having holes or voids through which the high energy rate pulse may be directly communicated to the surface of the part. The voids may be formed by ribs that form a honeycomb or other reticulated structure.
According to yet other aspects of the invention, the partially formed metal part may be formed to a preliminary shape which after spring-back is contoured with a gap being defined between the part and the forming surface of the die. The part may be stretched toward the final part shape to thereby eliminate the gap.
According to another aspect of the present invention, a method of calibrating a partially formed metal part is provided in which the part is clamped by an elastic membrane to a tool that provides a high rate energy pulse. A calibration die having a forming surface may be inserted into the elastic membrane so that the elastic membrane engages an opposite side of the part from the surface engaging the calibration die. A high energy pulse is provided to the elastic membrane and the opposite side of the part through the elastic membrane to relieve stress in the part. The pulse may also stretch the part onto the forming surface of the calibration die to overcome the spring-back effect inherent in the part.
According to other aspects of the invention, the elastic membrane may be provided in conjunction with an electro-hydraulic forming tool that has a chamber that contains a liquid and a plurality of electrodes that are retained within the tool at spaced locations. The electrodes may receive a capacitive discharge that results in a high energy pulse being applied to the elastic membrane and the part to thereby calibrate the part to a desired shape.
According to other aspects of the method, the method may also include forming a metal blank in an electro-hydraulic forming operation before it is processed further as a partially formed part in an electro-hydraulic calibration tool. The elastic membrane may be shaped generally to follow the contour of the opposite side of the part from the surface engaging the die.
The above described aspects of the invention and other features and advantages will be described below with reference to the attached drawings.
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If a gap 52 is provided between the part 30 and the target-forming surface 44, the part 30 may be stretched by the pulse.
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While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.
Claims
1. A method of calibrating a metal part that has been formed in a first direction to cause a first side of the part to protrude in the first direction, the method comprising:
- loading the part onto a forming surface of a die in an electro-hydraulic forming tool that has at least one electrode that creates a high rate energy pulse;
- clamping the part in the electro-hydraulic forming tool with the first side of the metal part facing the electrode; and
- applying the high rate energy pulse to the first surface to overcome a spring back effect in the part.
2. The method of claim 1 wherein the loading step further comprises loading the part into the electro-hydraulic forming tool having a chamber that is filled with a fluid, and wherein the applying step is performed by applying the high rate energy pulse that is in the form of at least one electro-hydraulic forming pulse that is transmitted to the first surface of the panel through the liquid.
3. The method of claim 1 wherein further comprising the pulsing step being performed on the first surface of the part to relieve stress in the part.
4. The method of claim 1 further comprising the pulsing step being performed on the first surface of the part and the part is stretched to eliminate spring-back in the part.
5. The method of claim 1 further comprising the part being formed in an electro-hydraulic forming tool before the loading step.
6. The method of claim 1 further comprising the clamping step being performed with a plurality of clamps that engage the first surface of the part and the high energy rate pulse is applied to exposed portions of the part not engaged by the clamps.
7. The method of claim 1 further comprising the part having a preliminary shape that after spring back is contoured with a gap being defined between the part and the forming surface of the die, wherein the gap allows the part to be stretched toward a final part shape.
8. A method of calibrating a partially formed metal part that has a first side that protrudes in a first direction, the method comprising:
- clamping the first side of the part into engagement with an elastic membrane;
- inserting the part and the elastic membrane into a tool that provides a high energy pulse and that includes a calibration die that has a forming surface, wherein the a first side of the part faces away from the forming surface; and
- pulsing the elastic membrane and the first side of the part with a high rate energy pulse to form the part against the forming surface to overcome a spring back effect in the part.
9. The method of claim 8 further comprising the elastic membrane being provided in conjunction with an electro-hydraulic forming tool that has a chamber that contains a liquid and a plurality of electrodes that are retained within the tool at spaced locations, wherein the electrodes receive a capacitive discharge that results in the high rate energy pulse being applied to the elastic membrane, and the first side of the part to calibrate the part to a desired shape.
10. The method of claim 8 further comprising the pulsing step being performed on the part and the part is stretched to eliminate spring-back in the part.
11. A an electro-hydraulic forming tool for forming a partially formed part to a final shape, the part having a first surface that protrudes in a first direction, the tool comprising:
- a vessel that contains a liquid;
- at least two electrodes disposed in the vessel and that are operatively connected to a capacitive discharge circuit;
- a one sided die that is supported by the tool in the vessel;
- a retaining ring that supports the partially formed part on the die with the first surface extending into the vessel; and
- at least one clamp that is assembled to the partially formed part and holds the part to the die, wherein a capacitive discharge from the discharge circuit causes the electrodes to provide a high energy rate pulse through the liquid to the first surface of the part to overcome a spring back effect in the part.
12. The tool of claim 11 wherein electrodes apply the high energy rate pulse to the part and wherein the part is stretched to eliminate spring-back in the part.
13. The tool of claim 11 wherein a plurality of clamps engage the part at spaced locations to define exposed portions of the part not engaged by the clamps.
14. The tool of claim 13 wherein the clamps are engage the part during the high energy rate pulse and wherein the clamps engage the previously exposed portions of the formed part with newly exposed portions being open to a second high energy rate pulse.
15. The method of claim 11 wherein the clamp is a reticulated clamp that includes ribs that define openings through the body of the clamp between the ribs.
16. A method of calibrating a partially formed metal part comprising:
- loading the part onto a forming surface of a die;
- clamping one side of the formed part onto the forming surface with a clamp so that portions of the formed part on a second side of the part that are not engaged by the clamp are exposed portions;
- pulsing the exposed portions of the part with a high rate energy pulse to overcome a spring back effect in the part; and
- wherein the clamping step is performed in a first clamping step with a plurality of clamps that engage the part at spaced locations and the high energy rate pulse is applied in a first pulsing step to exposed portions of the part not engaged by the clamps, and wherein a second clamping step is performed with the clamps being repositioned on the part on the previously exposed portions of the formed part with newly exposed portions being open to a second pulsing step with a second high energy rate pulse.
17. A method of calibrating a partially formed metal part comprising:
- loading the part onto a forming surface of a die;
- clamping one side of the formed part onto the forming surface with a clamp so that portions of the formed part on a second side of the part that are not engaged by the clamp are exposed portions;
- pulsing the exposed portions of the part with a high rate energy pulse to overcome a spring back effect in the part; and
- wherein the clamping step is performed with a reticulated clamp that includes ribs that define openings through the body of the clamp where the exposed portions of the formed part are open to the high energy rate pulse.
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Type: Grant
Filed: May 5, 2008
Date of Patent: Nov 9, 2010
Patent Publication Number: 20090272167
Assignee: Ford Global Technologies, LLC (Dearborn, MI)
Inventor: Sergey Fedorovich Golovashchenko (Beverly Hills, MI)
Primary Examiner: David B Jones
Attorney: Brooks Kushman P.C.
Application Number: 12/115,028
International Classification: B21D 26/12 (20060101); B21D 26/06 (20060101);